Various kinds of devices are used conventionally as line-waveguide converters for converting transmission signals between a signal line and a waveguide. For example, JP 8-139504A discloses a line-waveguide converter, in which a waveguide is excited by a patch antenna. Further, JP 6-112708A discloses another line-waveguide converter, in which a back short is used and a line is laterally disposed in the direction of signal propagation in a waveguide.